Telephone call distributing system



Nov. 3, 1936. J. MESZ A R 2,059,596

TELEPHONE CALL DISTRIBUTING SYSTEM I Filed Nov. 50, 1934 16 Sheets-Sheet l T TOPNEY Nov. 3, 1936. J. MESZAR 2,059,596 I I -TELEPHONE C ALL DISTRIBUTING SYSTEM Filed Nov. 30, 1934 16 Sheets-Sheet 2 ML w? F/G 2 PRIMARY SWITCH no. 2

PR/MAkY SWITCH N0.

Q" E9 INVENTOR J.MSZAR ATiORA/EV Nov. 3, 1936. J. MESZAR- TELEPHONE CALL DISTRIBUTING SYSTEM Filed Nov. 30, 1934 16 Sheets-Sheet 3 m/s/E/v TOR J. MESZA R A TOR/V5) NOV. 3, 1936. MESZAR TELEPHONE CALL DISTRIBUTING SYSTEM Filed Nov. 50, 1934 16 Shets-Sheet 4 SN mi llVl ENTOR JMESZAR InHl- WWW ATTORNEY Nov. 3, 1936. J. MESZAR 2,059,596

TELEPHONE CALL DISTRIBUTING SYSTEM Filed Nov. 50, I934 l6 Sheets-Sheet 5 SECONDARY SWITCH NO. IO

nvvmvrop J. M5524 I? ATT /VE V NOV. 3, 1936. r 4 MESZAR 2,059,596

TELEPHONE CALL DISTRIBUTING SYSTEM Filed Nov. 30, 1954 16 Sheets-Sheet 6 SECb/VDARY SWITCH no. 2

, TTORNEV Nov. 3, 1936. J. MESZAR 2,059,596

TELEPHONE CALL DISTRIBUTING SYS TEM Filed Nov. 30, 1934 16 Sheets-Sheet 7 SECONDARY SWITCH NO- I M/l/ENTOR J. MESZA R ATTOPAIFV Nov. 3, 19 36.

J. MESZAR TELEPHONE C'ALL DISTRIBUTING SYSTEM Filed Nov. 30, 1954 16 Sheets-Sheet 8 w QQQQQU kabkL I. .IiL aw //v VEA/ TOR J. MESZA Nammfl 19360 J MESZAR Q 2,059,596

TELEPHONE CALL DISTRIBUTING SYSTEM Filed Nov. 30, 1934 16 Sheets-Sheet 9 a I 1: Q

CCTS

New. 3, 1936. J. ME'szAR, 2,059,596

TELEPHONE CALL DISTRIBUTING SYSTEM Filed Nov. 50, 1934 16 Sheets-Sheet l0 Filed Nov. 30, 1954 16 Sheets-Sheet 11 W01 QMRO ko t8 qm Eta //v VE/VTOR J. ME 5 Z A R A T TORNE V J. MESZAR M95956 1e Sheets-Sheet 12 TELEPHONE CALL DISTRIBUTING SYSTEM Filed NOV. 30, 1954 Nov. 3, 1936..

OPER. TEL

Nov. 3, 1936. MESZAR 21%9596 TELEPHONE CALL DISTRIBUTING SYSTEM Filed Nov. 30, 1934 l6 Sheets-Sheet 13 OPER. TEL

SWITCH B0- OPE'R. P032 lNl/ENTOR J. MESZAR 8V A TTORA/EV Nov. 3, 1936. MESZAR 2,059,596

TELEPHONE CALL DISTRIBUTING SYSTEM Filed Nov. 30, 1954 16 Sheets-Sheet l4 SWITCH 50.

CCT 2 SWITCH B0.

CCT. 2

OPER. POS.

INVENTOR JMESZ'AR NQV. 3; 1936. Z I 2,059,596

TELEPHONE CALL DISTRIBUTING SYSTEM Filed Nov. 30, 1934 16 Sheets-Sheet 15 i I A v PHI-E i he. /5 B SWITCH 50.

CCT.

SWITCH 50.

CCT.

OPER. P05. 2

lNI/E/VTO/P J. MESZAR BY 1" v l ATTORNF NOV. 3, 1936. J ZA 2,059,596

TELEPHONE CALL DISTRIBUTING SYSTEM Filed Nov. 30, 1934 16 Sheets-Sheet 16 INVEN TOR J. MESZA R BY ATTO A/FV Patented Nov. 3, 1936 UNITED STATES PATENT OFFICE TELEPHONE CALL DISTRIBUTING SYSTEM John Meszar, Bogota, N. J., assignor to Bell Telephone Laboratories,

Incorporated, New

This invention relates to telephone systems and more particularly to what are known as call distributing systems whereby calling lines are automatically routed to idle operators.

An object of the invention is to extend an incoming calling line as, for example, a trunk circuit or toll line, to an occupied switchboard position at which the operator is free to answer the call.

A feature of the invention whereby the foregoing object is attained, resides in dividing the calling lines into groups, terminating the lines of each group in contacts of a primary cross-bar switch individual to the group, extending a plurality of link circuits from each primary switch to a group of secondary cross-bar switches, terminating each link circuit in contacts of one of the secondary switches, extending a plurality of circuits from each secondary switch to and distributing them among a plurality of operators positions and there terminated in answering terminals and, in an arrangement whereby a calling line causes certain circuit operations to take place including the following:

(1) Mark the calling line at the primary switch;

(2) Locks out subsequent calls after the system has started working on one or more calls until all the calls on which it is working have been connected to switchboard circuits;

(3) Tests a predetermined first choice switchboard position to (a) determine if the position is occupied, (1;) determine whether the operator is idle, determine whether the position has an idle answering terminal available to one of the secondary switches associated with the primary switch in which the calling line terminates and if no idle terminal is. found continues testing until a second, third, etc. choice position is found in which an idle terminal appears to which the call can be routed;

(4) Causes the position to initiate a hunting operation to find the calling line and connect it, by means of the primary switch, to the lowest numbered idle link circuit extending to a secondary switch having access to an idle connecting terminal appearing in the chosen position and to cause the secondary switch to connect the link circuit thereto.

Other features include the following:

The link circuits from two or more primary switches can be multipled thereby reducing the number of link circuits available for extending a given group of calling lines. In like manner the outgoing switchboard circuits from two or moresecondary switches can be multipled thereby efiecting further economy by reducing the number of answering terminals available to the calling line group.

The foregoing is possible due to the fact that in the present system both the link circuit and the answering terminal (switchboard, plug or jack circuit) are common equipment with respect to the calling line and reduction by means of multipling can be carried out until only sufiicient link circuits and answering terminals are available for satisfactory service.

In any group of calling lines served by a corresponding group of link and switchboard circuits, only one calling line is hunted for at a time.

Calling'lines are hunted for in their numerical order in the group, i. e., the lowest numbered is answered first, the next higher numbered next, etc.

If a calling line has been hunting but cannot be connected through to the switchboard due to an all links busy condition or for some other reason, a timing arrangement causes the system to drop working on the call and pass on to the next. Under this condition, the dropped call is not given attention until the system has passed through its cycle and again reaches the line in its regular order of sequence.

All idle positions are notified when a call arrives. One position is assigned to take the call and all others are released at once even though more than one call has come in. If simultaneous calls had arrived, the operators will return after they have tested other groups.

A position will not be assigned to hunt for two successive calls unless it is the only position free to do so.

The preferred embodiment of the system of the invention in which the foregoing and other objects are attained employs cross-bar switches in a primary secondary switch arrangement. As shown in the accompanying drawings in which Figs. 1 to 16, inclusive, when placed together as indicated in Fig. 1A illustrate the method of extending a calling toll line or trunk to an available plug circuit before an idle toll operator. For clearness in reading, the description which following the talking circuit of a typical calling trunk and its extension is shown by medium heavy lines and the numerous series relay control circuits are shown by heavy black lines.

The calling toll line or trunk shown in Fig. 1 represents one of one hundred similar trunks each connected to a vertical row of contact springs of one primary cross-bar switch I unit,

shown in Fig. 2. Within the switch, each set of vertical springs may be connected to any one of ten horizontal sets of springs. The horizontal springs of each primary switch 1 unit are directly connected to a vertical row of contact springs on each of ten secondary cross-bar switch units shown in Figs. 5, 6 and '7. Within each secondary switch unit, each set of vertical springs may be connected to any one of ten horizontal sets of springs, each of which are connected to switchboard circuit shown or indicated in Figs. 9, 10, 11 and ending in ten plugs shown in Figs. 9 and 13 before each of ten operators. Every group of one hundred incoming trunks thus requires ten primary switch units of which the first, second and tenth are shown in Figs. 2 and 6 mounted on what is generally termed a primary bay. According to the embodiment of this invention, the simple case is taken for illustration of providing one hundred links between a primary and its associated secondary bay. A group of one hundred switchboard circuits are also provided for serving the secondary bay if the traffic ratio is 1:1. A 2:1 reduction between a group of incoming trunks and switchboard circuits may be made by connecting one primary bay in multiple with another primary bay. The second bay in this case is called a supplementary primary .bay. The switchboard circuits likewise may be connected to the horizontal springs of switches of a second, third, etc. secondary bay to obtain various ratio reductions between links and switchboard circuits. These bays are referred to as multipled secondary bays. By the use of supplementary primary bays and multipled secondary bays, a group of one hundred switchboard circuits may serve, according to traffic requirements, any number of incoming trunk circuits in multiples of one hundred. Odd ratios between incoming circuits and switchboard circuits may also be obtained by connecting less than one hundred incoming circuits to a primary bay or less than one hundred switchboard circuits to a secondary bay.

The cross-bar switches illustrated in Figs. 2, 5, 6 and '7 are of a well-known type and the specific structure and maintenance thereof is described in detail in the copending application of J. N. Reynolds Case '76, Serial No. 702,453 filed December 15, 1933. For purposes of this invention, it is sufficient to remember that the connection between any vertical and any horizontal contact spring is determined by the operation of the corresponding hold magnet I-ID when one of the selecting magnets S is energized, the latter magnet being thereafter released.

The system of this invention employs a line finder method of connecting an incoming trunk circuit to a switchboard circuit, that is, through the medium of group and position allotter circuits a number of switchboard circuits are selected capable of being connected to corresponding link circuits. A link allotter circuit then allots an idle link for connecting one of the switchboard circuits to the incoming trunk circuit. In order to start this selective action, restrict selection to the group of incoming trunk circuits that are next to be served, and control the hold and selecting magnets of the cross-bar switches, it is necessary to provide control relays and allotter circuits consisting principally of relays with series wiring. For example, incoming trunk circuits such as shown in Fig. 1 are connected in groups of ten to primary switch units such as shown in Fig. 2 with a group of ST control relays for each switch unit as shown in Figs. 3 and 7. These relays serve to determine the order of handling calls originating in a given unit one at a time. Another set of CH control relays with series wiring leads 2M, l2] and E32 serve to determine the order of handling calls originating from the ten different switch units one at a time. Therefore, out of all calls originating at a time in a given group of one hundred trunks only one at a time can be connected to a switchboard circuit and operator. The function of the link allotter circuit, shown in Fig. 4, is therefore to take these incoming calls and by means of the L, C and D control relays with series wiring in leads 223 and 226, determine and allot one of the link circuits through the primary and secondary crossbar switches for connecting the incoming trunk with a switchboard circuit and operator.

The group allotter circuit, shown in Fig, 8 with series wiring leads 426, 62! and 328, determine the order in which incoming calls from diiferent trunk groups such as 429, .30 and 43i are to be served. The position allotter circuit shown at the bottom of Figs. 12 and 16 with series wiring leads M6, 63! and 653 determines the order in which the operator positions are assigned to handle incoming calls. Groups of A, B and C control relays are shown in top portion of Fig. 12 and serve as signaling means between the group allotter, position allotter and switchboard circuits for position I. Control relays wired identically as shown in Fig. 12 are indicated by squares H5, 189 and T82 of Fig. 16 for position 2 and by squares 711, I83 and 184 for position ID. A typical switchboard circuit for position I is shown in Fig. 11 with similar circuits indicated by small squares such as 598 in Fig. 9 and 528 in Fig. 10. Switchboard circuits of other positions 2 to ID are indicated by similar squares in Figs. 13, 14 and 15. A set of control relays ST and LS associated with each switchboard circuit of the chosen operators position, shown in Figs. 9, 10 and 11, serves to insure that only one switchboard circuit at a time in the position will be connected to the operators telephone. Normally, the allotter and control circuits would prevent more than one call reaching an operator at a time but there are times when the operator receives a rering on an existing connection just at the moment her position is selected by an incoming call.

The primary and secondary cross-bar switches shown on the drawings and the link allotter circuit of Fig. i represents sufiicient equipment to care for one group of one hundred incoming trunk circuits. Similar cross-bar switch equipment, although not shown, is assumed for the purpose of this description for four other groups of one hundred trunks as indicated by the squares 432 and 433 in Fig. 8. Referring to Fig. 12, it will be noted that leads 6!)! and 604 connect with the switchboard circuits shown in Figs. 9, 10, 11 and 11A for position I. It is to be assumed that position I would have ten more plugs connected to switchboard circuits to care for the second group of one hundred incoming trunks, all wired and connected same as shown in detail for the first group. Leads SE12 and (-295 of Fig. 12 would, therefore, connect to similar points in the second group as shown for leads EDI and 604. Similarly for a fifth group of one hundred incoming trunks, a fifth set of ten switchboard plugs would be placed in position I with leads 603 and 606 of Fig. 12 connected to similar points as shown for leads 6B! and 694. In like manner, positions 2 to It! would have additional plugs so that it may be cuit shown in Fig. 8 i only one of the above A, B, C relay sets to function ondary cross-bar switches.

assumed in this description that the ten positions will each have five groups of ten switchboard circuits and plugs. For the purposes of this description, this 1:1 ratio of links to switchboard circuits is the simple case but it will be readily understood that this ratio is entirely flexible and dependent on the trafiic requirements for a particular office. Since five groups of ten switchboard circuits per position are assumed for this description, there will be five sets of A, B and C relays per position, shown or indicated in Figs. 12 and 16. Since each set controls its respective switchboard circuits, a group allotter ciris provided which will allow at a time on a given call. The wiring of the group allotter circuit, as will laterbe described, functions also to see that one call out of each group of one hundred incoming trunks is served before permitting a second call in any group to be served. In this way all groups of trunks have an equal chance at reaching an operator.

A call originating in the first trunk group operates the corresponding AGi, MG; and HG1 relays of Fig. 8 as will later be described so that if calls are also originating in other trunk groups the group allotter circuit will cause the calls to be handled in a definite sequence, one at a time. The position allotter relays HP, MP and AP of Figs. 12 and 16 then determine which position is to take the incoming call. If, for example, all positions were idle then position i would be chosen due to the series circuit over leads hi6 53! through the contacts of the HP relays as will later be explained. The idle switchboard circuits of the chosen position are then grounded or marked by the operation of relay 55-? of Fig. 11A common to position i, except those that are busy which have their relays 555 operated. Grounding the switchboard circuits makes it possible for the link allotter circuit of Fig. l to operate any one of the corresponding S magnets on the secondary cross-bar switch. thus extending an idle switchboard circuit over an available link such as so in Fig. '7 to the primary switch bays serving the group of one hundred incoming trunks in which the call has originated. Thus, it will be evident that any one of ten of the idle switchboard circuits of the chosen position may receive the incoming call, depending on which link is available. It is further obvious that several trunks in this particular group of one hundred may be calling at the same time so that the ST and CH series relay control circuits are provided for sorting out and marking a definite order, to the link allotter circuit of Fig. i, in which the calls are to be handled.

According to the foregoing general description, only one calling trunk at a time is capable of being extended by the link allotter circuit of Fig. l to any one of a number of idle switchboard circuits in the chosen position. One function of the link allotter circuit, therefore, is to select an idle path or link between the primary and sec- That is, it must match one of the idle switchboard circuits with an available link. In Figs. 5, 6 and 7, for example, any one of ten links represented by lines tit, i2 and d3 may connect with any one of switchboard circuits i to iii of position i and in the event no match is found, this circuit temporarily shelves the call, allowing other waiting calls in the groupto be served. This allotter circuit allows only one match to be made at a time between an idle switchboard circuit and an available link. The

primary secondary switch circuit is further controlled by the allotter circuit at relay m2 of Fig. 3 to admit all waiting calls to preferential service at the finish of the last call in the preferential group. Thereafter, all new calls must wait until the preferential calls have been served. The link alloter circuit further acts to control and time the cutting through of the incoming trunk call to an idle cord in the chosen operators position and to bring in an alarm in case of failure of any of the common leads or of the allotter itself.

The manner in which the system is organized will appear from the description of its operation which now follows. Let it be assumed that the incoming trunk i of Fig. 1 is originating the call, in which case the calling operator at the distant ofiice is sending out ringing current over the trunk in the usual manner, causing ring-up relay 5 to operate in a loop circuit traced from lead 6 through winding of repeating coil 1 over leads 8, 9 and is, winding of relay 5 in series with condenser il, leads l2 and i3, upper winding of repeating coil l, and back over lead M to the incoming trunk. Relay 5 closes two circuits, one from ground at its inner make contact through winding of relay 1% to battery which causes the latter to operate but which has no function at this time. The other make contact on relay 5 connects ground to winding of relay H to battery, causing the latter to operate and prepare one path from battery on the lower contact, through winding of relay l8 and lead 2, leading to primary cross-bar switch i of Fig. 2 for supervision purposes; and another path from ground on the upper contact over lead i to the primary switch l of Fig. 2 for starting purposes as will presently be described. No further operation of the trunk in Fig. 1 occurs now until the trunk is connected to an operator.

Let it further be assumed that the incoming trunk connected to the leads designated trunk ll] of Fig. 2 is also originating a call at this time. Ground over leads 1 and 3! and over leads 32 and 33 will then operate relays ST1 and STm of Fig. 3. Relay ST1 is operated over lower break contact of bold magnet HD1 and lead 3 1 of Fig. 2, thence over lead 3 1 of Fig. 3, lower winding and lower break contact of relay ST1, common lead iii! and break contact on relay N12 to battery. Relay STm is operated over asimilar path to the same common lead it)! and battery on the break contact of relay E92. Both ST relays being operated, substitute battery through left break contact of relay E23, leads 1M and Hi5, and contacts Hi6 and fill, for the battery over common lead ,lill. Thus, calls coming in on other trunks in the same unit are prevented from operating their ST relays until the two calls being traced have reached an operator. The upper break make contacts on. the ST1 and STm relays are now operated but have no function at this time since none of the selecting magnets S have as yet operated.

Both ST1 and ST10 relays at contacts 508 and its now feed battery to common lead lit which continues over leads ill and H2 for operating relay CH1 to ground at test jack M3. As previously referred to, all ten CH relays have their right-hand set of contacts connected in a series circuit arrangement over leads l2? and i31 which serves to determine the order in which the preferential calls in different cross-bar switch units are served. For purposes of this description, therefore, it is assumed that a third call on trunk 95 in primary switch ii! of Fig. 6 also came in with the two calls in primary switch I and that its respective CH10 relay also operates in unison with relay CH1. Both relays at their left contacts connect ground to two different circuits; one over multiple lead II4 which causes the group and position allotter circuits to ground or mark all idle switchboard circuits in a chosen position as will later be described; the other over multiple lead I I5 of Fig. 3, winding of relay 204 of Fig. 4, to battery causes the latter relay to operate and at its left make contact operate relay B2 for alarm purposes. At contact 205, relay 204 connects a path from ground through relay 200 to lead H0 and winding relay I02 of Fig. 3 to battery. Relay 200 of Fig. 4 operating, operates relay B5 for alarm purposes and relay I02 of Fig. 3 operating, removes battery from lead NH and those leads corresponding to lead IOI of each primary switch unit. In this manner, the further operation of any ST relay in other cross-bar switch units is prevented until the three trunk calls being traced have reached an operator.

Going back to relay 204 of Fig. 4, this relay also closes a circuit at its outer right make contact for operating relay 20'! and all of the C1 to C10 relays. This path is traced from ground at left back contact relay 200 in lower right corner of Fig. 4, thence over leads 200 and 2 I0, left make contact relay 204 to battery through winding relay 20! and also over lead 2 I I, lower back contact relay 2 I2, to battery through the C1 and C10 lower relay windings to battery. Relay 20'I operating, closes ground to left winding relay 2 l 3, thence on lead 2I4 and because relay CH1 of Fig. 3 is oper ated at this time, the circuit is closed at contact I I! and through winding relay I03 to battery. Re-

lay CH10 of Fig. 7 is also energized at this time out its associated relay 31S cannot operate on account of the series circuit through the CH relay contacts. Relay 2I3 of Fig. 4 functions only as an alarm relay While relay I03 of Fig. 3 has three functions. In the first place, at its left break contact it removes battery from multiple leads E04 and I05. This does not, however, release the ST relays associated with primary switch I because normally battery is also fed to lead I05 over lead H8 and back contact relay H0. As will later be explained, relay II9 operates only when the link allotter cannot match an idle switchboard circuit with an idle link or when calls from all one hundred incoming trunks are blocked. Secondly, at contact I20 it supplements the battery already on lead III and winding relay CH1, making holding of relay CH1, independent of relays ST1 and ST10. Lastly, at contacts I2I to I23, it prepares ten different paths for the operation of the selecting magnets on primary switch I of all ten connecting circuits called links.

One path through selecting magnet S10 in Fig. 2 for example, may be traced from battery through back contact of holding magnet I-ID1 of secondary switch I, shown in Fig. 7, thence over lead 35 to selecting magnet S10 of primary crossbar switch I of Fig. 2. Passing through winding S10, the path is further traced over lead 30 to contact I23 on relay I03 of Fig. 3, thence over common lead I25 to lower make contact of relay 2I5 of Fig. 4 and left break contact of relay D10. The other paths may be traced in a similar manner, each one starting from battery at the break contact of the holding magnets HD of secondary switch units of Figs. 5, 6 and 7. Whenever a link such as 42, for example, is in use, its corresponding holding magnets HD1 of Fig. 6 is operated as will later be described, so that battery is remove-d from lead 35I of Fig. 6, leading over link 42 to the corresponding selecting magnet S2 of the primary switch I of Fig. 2, thus making the link busy.

The description thus far has assumed that three calls have originated simultaneously; one from the incoming trunk of Fig. 1; one from the incoming trunk I0 indicated in Fig. 2 and one from trunk indicated in Fig. 6. Because of the series circuit through the right contact springs of the CH relays over leads I21 and I32 it is obvious that although both relays CH1 and CH10 may be operated, still only the first of the relays I03, H25 of Fig. 3 and relay 310 of Fig. 7 can be operated as above described. This relay I03, therefore, connects ten common leads of Fig. 4 such as I 26 to primary switch unit I, thereby eliminating for the moment the call on trunk 9|. The choice of a link leading to a. secondary switch unit is further restricted to the incoming trunk of Fig. 1 by the ST relay series of Fig. 3. Thus, when ground from the front contact of whatever selecting magnet is energized is later connected to lead 39 in Fig. 2 and Fig. 3, the holding magnet HD1 will be energized through the top front contact of relay ST1 and the call on trunk IO whose ST10 relay is also operated will have to wait until the first call is connected to an operator. The relays involved in this set-up that have functions as described therefore remain operated until the group allotter circuit of Fig. 8 and the position allotter of Figs. 12 and 16 function to pick out or mark the idle switchboard circuits of a chosen operator position and the link allotter of Fig. 4 functions to match one of the switchboard circuits with an available link which will connect with the incoming trunk of Fig, 1 at primary switch 5 of Fig. 2.

The following description will serve to show more clearly how the group allotter circuit of Fig. 8 and position allotter circuit of Figs. 12 and 16 function. These circuits are so arranged that the operator telephone sets 50I of Fig. 9 and 'l'0I and 103 of Fig. 13 are required to be plugged into the position jacks 502, I02 and I04 in order to make the switchboard circuits of the positions selectable. Tracing, for example, from ground on the make contact of telephone jack 502 of Fig. 9 through break contact on relay 503, lead 504 of Figs. 9, 10 and 11, through relay 55! of Fig. 11A to battery, a path is completed for operating relay 55I which in turn connects battery through break contact on relay 552 to lead 553 of Figs. 11 and 12 connected to the windings of every A relay corresponding to each group of one hundred incoming trunk circuits. A further requirement for making an operator position selectable is that there be some switchboard circuit available for receiving a trunk call. This is provided for by connecting ground to lead 604 of Fig. 12 for the A relay to operate over the path previously traced to battery. Ground to lead 004 of Figs. 9, 10, 11 and 12 is supplied at back contact 554 on relay 555 of Fig. 11 and corresponding contacts on similar relays not shown in Figs. 9 and 10. Relay 555 remains operated whenever the switchboard circuit is busy, due to its winding being connected over lead 556 to secondary switch I0 of Fig. '7, lead 26 to primary switch I of Fig. 2, lead I and ground at incoming trunk of Fig. 1. The topmost A relay of Fig. 12, therefore, will not operate if all the switchboard circuits that answer calls in the first group of one hundred incoming trunks are busy. Assuming, however, that switchboard circuits are 

